Meaning: 'Acceptable' means that something is considered satisfactory or suitable, indicating that cells are recognized as a valid concept in biology.

Wrong options:

• determined: 'Determined' implies a sense of resolution or decision, which does not fit the context of cells being a unifying concept.
• interactive: 'Interactive' suggests engagement or communication, which is not relevant to the description of cells as a concept.
• claimed: 'Claimed' indicates a declaration or assertion, which does not appropriately describe the nature of cells in this context.

Correct answer: unit

Explanation: The term 'unit' is grammatically correct as it refers to the smallest component of structure and function, fitting perfectly in the context of biological terminology. It conveys the idea of cells being fundamental building blocks.

Meaning: 'Unit' refers to a single, complete entity that can function independently, in this case, a cell as the smallest part of living organisms.

Wrong options:

• object: 'Object' is too vague and does not specifically convey the biological context of cells as functional components.
• value: 'Value' refers to worth or importance, which does not apply to the structural aspect of cells.
• role: 'Role' implies a function or position, which does not accurately describe the physical nature of cells.

Correct answer: so

Explanation: The word 'so' is used here to indicate a degree, fitting grammatically and contextually to emphasize the small size of individual cells. It serves to strengthen the statement about their visibility.

Meaning: 'So' means 'to such a great extent' and is used to express the idea that cells are very small, reinforcing the statement about their visibility.

Wrong options:

• quite: 'Quite' suggests a moderate degree, which does not convey the same emphasis on the smallness of cells as 'so' does.
• now: 'Now' indicates a current time frame, which is irrelevant in this context about the size of cells.
• also: 'Also' implies addition, which does not fit the context of describing the size of cells.

Correct answer: for the first time

Explanation: The phrase 'for the first time' is grammatically correct and contextually appropriate, indicating the initial observation of cells by Robert Hooke. It aligns with the historical significance of the event.

Meaning: 'For the first time' means that this was the initial occurrence of observing cells, marking a significant moment in scientific history.

Wrong options:

• for the earliest time: This phrase is awkward and not commonly used in English, making it less suitable than 'for the first time.'
• at the beginning: 'At the beginning' is too vague and does not specifically indicate the initial observation of cells.
• in the beginning: Similar to 'at the beginning,' this phrase lacks specificity and does not convey the historical context effectively.